Radio-frequency identification (RFID) systems are well-known and used for identifying and/or tracking equipment, inventory, or living things. Basically, an RFID system is a radio communication system comprising a radio transceiver, referred to herein as an interrogator, and a number of inexpensive devices called tags or transponders. Conventional RFID systems are designed to read the information contained on a tag when it passes within range of an interrogator. This type of system is called a passive system because information is only read from the tag and not written to it. Another type of RFID system is called an active system. This type of system is one in which an interrogator can both read the data on a tag as well as write new information or overwrite existing information on the tag when it passes within the range of the interrogator. RFID systems are described, for example, in U.S. Pat. Nos. 6,255,993 and 6,184,841.
In a typical RFID system, the interrogator communicates to the tags using modulated radio signals, and the tags respond with modulated radio signals. A common RFID system is a modulated backscatter (MBS) system. In an MBS system, after transmitting a message to the tag (called the downlink), the interrogator then transmits a continuous-wave (CW) radio frequency (RF) signal to the tag. The tag then modules this CW RF signal in accordance with data specific to the tag and transmits it back to the interrogator. Therefore, modulated backscatter allows communications from the tag back to the interrogator (called the uplink). Another type of RFID system uses an active uplink (AU). In an AU system, the RFID tag does not modulate and reflect an incoming CW signal, but rather synthesizes an RF carrier, modulates that RF carrier, and transmits that modulated carrier to the interrogator. In some AU systems, the RF carrier used in the uplink is at or near the same frequency as that used in the downlink, while in other AU systems, the RF carrier used in the uplink is at a different frequency than that used in the downlink.
An important component of an RFID system is the antenna system located in the interrogator. The antenna system functions to convert data between electrical signals and electromagnetic radiation, To this end, a typical RFID system comprises a transmitting antenna which converts electrical signals to electromagnetic fields of a certain pattern on the surface of the antenna to facilitate electromagnetic, or, more specifically, RF, radiation, and a receiving antenna which absorbs RF radiation to create electromagnetic fields on its surface and convert these fields back into electrical signals.
As with most electronics, there is a need to miniaturize RFID systems to save space and cut cost without sacrificing performance. Miniaturizing the antenna system of an RFID system, however, has proven difficult since isolation between the transmit and receive antennas, which is critical for good performance, tends to be compromised as the antenna are moved closer together. One approach for improving isolation between closely-mounted antennas involves using a partition between them. For example, the antenna systems sold by ThingMagic (Cambridge, Mass.) and Symbol (formerly Matrics of Columbia, Md.) have adequate isolation between the antennas, but use a partition between the antennas which consumes valuable space and therefore contravenes the objective of miniaturization.
Therefore, there is a need for an RFID antenna system that facilitates miniaturization while maintaining good isolation. The present invention fulfills this need among others.